The welding of components from polymer material has been known for a long time. It enables complex assembled structures to be built up from individual components. Welding is used in the manufacture of components for motor vehicles, for example.
In that case, components with large dimensions, e.g., bumper claddings made of polymer material or wings made of the respective sheet-like parts forming the external skin and of structural elements that serve for reinforcement, for example, are joined together by welding. The structural elements are also used for shaping, reinforcing and fastening the components forming the external skin. Here, very high requirements are set in respect of dimensional accuracy and the appearance of said components.
Welding is generally accomplished by way of “welding domes,” i.e., lugs which are arranged on one part and pass through the corresponding apertures in the second component and, in this state, connect it by a riveting action by way of ultrasonic welding or ultrasonic riveting, for example, through a change in shape in the softened state. However, there are also welding techniques such as vibration welding, rotary welding or welding by way of the action of heat or radiation which are likewise used in this case.
In this context, German Patent Publication No. DE 20 2005 013 901 discloses a method which welds individual ridges.
Problems have always occurred, however, with the known welding method when coated components are to be connected.
As a solution for connecting at least partially coated outer shells to inner shells, adhesive bonding is chosen in the prior art, but a pre-treatment of the component surface is generally necessary as well.
In addition, adhesive joints represent a weakening of the stiffness of the component since the adhesives can only have a low strength and rigidity and, as a result, the joint is compromised mechanically at the adhesive bonding point. PP, that is to say polypropylene or polypropene, is widely used in vehicle construction. Large-area structures in particular, such as the internal trim of vehicles but also tailgates and doors, are produced from this plastic.
There are several welding methods available for welding PP, e.g., hot element welding, ultrasonic welding or laser welding. The principle behind these welding methods is that the welding zone is softened, i.e., converted to a state in which it can be shaped, by supplying energy from the outside, this being a method in several stages since heating and joining do not take place simultaneously. The method envisages that heating elements heat the connection points of the plastic parts until they reach the melting temperature. The connection points are then pushed on to one another and pressed together until they cool fully.